fix(pipstar): correctly handle platlib and purelib in .data

[aignas]

Some packages like use platlib in the data to put the main files. This
PR is implementing correct handling of such packages by recursively
merging two trees. If we have any collisions, we will print an error
and stop. That is unlikely but better to be safe. Users can patch the
failure to be a warning if necessary.

In order to make this more testable, move the functions to a separate
file.

Fixes #3500
Fixes #2949
To be cherry-picked as part of #3466

[gemini-code-assist]

Summary of Changes

Hello @aignas, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the pipstar functionality by introducing a robust method for handling Python packages that utilize platlib and purelib directories within their .data section. The primary goal is to ensure these components are correctly integrated into the installation path through a recursive merging process, preventing potential file conflicts. The extraction logic has been refactored into a new Starlark file for improved organization and maintainability, and an example has been updated to validate the new behavior.

Highlights

• Correct handling of platlib and purelib in .data: Implemented a recursive merging mechanism for directories within the .data section of Python wheels, specifically for 'platlib' and 'purelib' components, to correctly place them into 'site-packages'. This resolves issues where some packages (like libclang) use these directories to store main files.
• Collision detection: The new merging logic includes collision detection, which will result in an error and stop the process if conflicting files are found during the merge. This ensures data integrity, with an option for users to downgrade to a warning if needed.
• Refactoring of wheel extraction logic: The core wheel extraction logic, previously in whl_library.bzl, has been moved to a new, dedicated Starlark file whl_extract.bzl. This improves modularity and testability of the extraction process.
• Example update for testing: The libclang package has been added to the examples/pip_parse project to provide a concrete test case for the new platlib/purelib handling functionality.

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[gemini-code-assist]

Code Review

This pull request correctly handles platlib and purelib in wheel .data directories by implementing a tree merge logic, which is a solid improvement over the previous simple rename approach. The refactoring of this logic into a new whl_extract.bzl file also improves code organization. I've found a couple of potential issues in the new merge_trees function that could lead to a crash or silent failures and have provided suggestions to make it more robust.

[rickeylev]

re: error on collision: what do other installers do?